Band reject filters

ABSTRACT

Acoustic resonators such as surface acoustic wave (SAW) devices and thin film bulk acoustic resonators (FBAR) can be configured to produce a band reject filter. Such a filter overcomes the insertion loss and power handling limitations of conventional band pass configurations and as such can be used in power amplifier and duplexer applications.

FIELD OF THE INVENTION

[0001] This invention relates to an RF band reject filter, to a basestation power amplifier for a cellular radio network, to a duplexer fora mobile telephone handset, and to a low noise amplifier (LNA) inputstage.

BACKGROUND OF THE INVENTION

[0002] Surface acoustic wave (SAW) devices have been studied andgradually commercialised since the mid 1960s. Such devices typicallyhave electrodes in the form of interlocked “fingers” (so-calledinter-digital electrodes) formed on a piezoelectric substrate. When highfrequency signals are applied to the input electrodes, mechanicalvibrations in the form of travelling acoustic waves are generated in thepiezoelectric substrate which are picked up by the output electrodes.Generally speaking, when the wavelength of the surface acoustic wavesand the period of the electrode “fingers” are the same, the magnitude ofthe surface acoustic waves are at their greatest and the device has alow electrical impedance. At other input frequencies, the device appearsto have a higher electrical impedance.

[0003] Thus, such a so-called “SAW resonator filter” can be made to havea very precise and narrow (typically having a Q factor over 1000), bandpass characteristic. Furthermore, since surface acoustic waves travelacross the substrate 100000 times more slowly than the speed ofeleotromagnetic waves, such devices are generally compact. In practice,such devices can be used in a ladder configuration (with a plurality ofshunt and plurality of series resonator filters used together). Thisallows a combined band pass characteristic to be tuned as desired.

[0004] Thus such devices have found many uses. However, such devicessuffer from two significant disadvantages which prevents their use insome applications. Firstly, band pass filters produced using SAWresonators typically have relatively high insertion losses typically ofa minimum of 1 or 2 dB. The state of the art presently is an insertionloss of about 1 dB in the pass band with a rejection of about 15 dB inthe stop band for a single stage band pass ladder filter. The lossestypically occur as a result of visco-elastic attenuations and/or modeconversions from SAW to bulk acoustic waves when the electrical energyis converted to acoustic energy and travels around the SAW filtercavity. Secondly, the power handling capability of SAW filters islimited. At high powers, the ultrasonic vibration to which the metallicelectrodes are subjected eventually causes the metal grain boundaries tomigrate. Thus, for example, at the present 1800, 1900 and 2100 MHzcellular mobile bands, such filters cannot be used for a mobile handsetduplexer because at these frequencies, such filters cannot survive for arealistic length of time at the desired power levels of approximately 30dBm.

[0005] Relatively little work has been done on SAW notch or band rejectfilters to date. Of the little work which has been reported, most of ithas focussed on the development of narrowband notch filters. One of thefirst publications on SAW notch filters was in U.S. Pat. No. 4,577,168(Hartman). Various techniques for implementing SAW notch filters aredescribed in which the conductance within the passband of a Single PhaseUnidirectional Transducer (SPUDT) SAW transducer was used as animpedance element to create a notch filter. One implementation used theimpedance of a SPUDT in conjunction with an RF transformer and otherimplementations consisted of replacing the capacitors in a bridge-T typenotch filter with a SPUDT transducer impedance element. This approachhas one disadvantage in that SPUDT transducers fall into the class ofFinite Impulse Response Devices and hence the device must be made longerif narrow notch bandwidths are to be achieved. Furthermore, SPUDT typedevices are not easily manufactured at elevated frequencies since ⅛wavelength electrodes are required.

[0006] A variation of this technique is described in S. Gopani and B. A.Horine “SAW Waveguide-Coupled Resonator Notch Filter”, UltrasonicsSymposium, 1990, in which a Two-Pole Waveguide Coupled (WGC) Resonatoris embedded in an all pass network to implement a notch filter. Thistechnique has two major disadvantages. Firstly the WGC resonator islimited to Quartz hence only bandwidth of 0.1% are attainable andsecondly, the resonator has a very poor shape factor of around 5.3 sincea typical device might have a 40 dB stopband width of 84 kHz and the 3dB stopband width of 444 kHz. The device described had a centrefrequency of 247 MHz and the insertion losses in the passband were ofthe order of 4 dB.

[0007] A further modification is described in P. A. Lorenz and D. F.Thompson, “Wide Bandwidth Low Cost SAW Notch Filters”, UltrasonicsSymposium, 1998. This technique consisted of placing two single pole SAWresonators in series with a resonator in between them. This techniqueachieved notch depths of more than 40 dB but had a relatively poor shapefactor of 4.3 where the 40 dB stopband width was 86 kHz and the 3 dBstopband width was 370 kHz at a centre frequency of 420 MHz. Insertionlosses in the stop band were approximately 5 dB or less.

[0008] Other simpler implementations consist of using a single pole SAWresonator in series with the signal to obtain a notch at theanti-resonance frequency. Although simple, this filter has a relativelynarrow rejection bandwidth, and the shape factor is very poor.

[0009] All the reported SAW notch filter developments focused on narrowband notch filters versus wider band reject filters. Furthermore, thetechniques consisted of using the impedance of a SAW SPUDT or resonatordevice in an all pass network to generate a notch response near thepassband of the SAW device and leveraged the capacitive properties ofthe SAW device away from the notch to form an all pass network. Ratherpoor notch shape factors and insertion losses have been achieved in thereported literature. Therefore there is a need for wider rejection banddevices and or lower insertion losses within the passband.

SUMMARY OF THE INVENTION

[0010] According to a first aspect of the invention there is provided anRF band reject filter comprising a shunt acoustic resonator and a seriesacoustic resonator, the shunt resonator being arranged to resonategenerally at the reject frequency band and the series resonator beingarranged to be anti-resonant generally at the reject frequency band.

[0011] The precise placement of the series resonator anti-resonancefrequency and shunt resonator resonance frequency can be adjusted toachieve a desired tradeoff between rejection band depth or rejectionband width as explained in detail below.

[0012] This arrangement overcomes the power handling problem.Conventionally, acoustics resonators are configured to provide a bandpass configuration. In this configuration, the series resonators arechosen to be at resonance in the pass band and the shunt resonators arechosen to be at anti-resonance in the pass band. However, by reversingthis configuration, a notch or band reject filter is produced.

[0013] Such a band reject filter could be used, for example, at a poweramplifier output in a base station or handset transceiver with thereject band being tuned to the receive band noise. The filter willreadily allow a large transmit signal to pass since the filter is tunedto the receive band and it is thus at receive frequencies that strongacoustic resonance occurs. Since the receive band power is low, andsince in the transmit band there is virtually no acoustic resonance,acoustic-electric migration is not a significant factor in such afilter. Thus, the power handling of such a device in the transmit bandis limited by electro-migration (and eventually arcing between theelectrodes). This occurs at much higher power levels than the powerlevels which may be passed by a prior art band pass acoustic resonatorfilter at its pass band frequency.

[0014] Furthermore, in this band reject configuration of the presentinvention, the filter appears as a high Q series and shunt capacitoroutside its reject band. By including a high Q matching network, thecapacitance (outside the reject band) can be substantially reduced sothat insertion losses in the region of 0.2 dB are attainable for afilter having 15 dB attenuation in the reject band.

[0015] Thus, for example, such a filter may be used in an UMTS basestation in order to reduce the cost of the duplexer module (whichpresently makes up about 10% of the total cost of the base stationcabinet).

[0016] Thus, in a second aspect, there is provided a base station poweramplifier for a cellular radio network, the power amplifier including atleast one inter-stage band reject filter comprising a shunt acousticresonator and a series acoustic resonator, the shunt resonator beingarranged to resonate generally at the reject frequency band and theseries resonator being arranged to anti-resonant generally at the rejectfrequency band.

[0017] The use of the band reject filter of the present invention as aninterstage filter for a power amplifier is particularly suitable sinceas noted above, the band reject filter of the present invention appearsas a high Q series and shunt capacitor outside the rejection band. Theequivalent capacitor has a short delay and therefore produces a verywideband characteristic with good phase linearity and amplitudeflatness. This is in contrast to pass band resonators which have aparabolic group delay and hence poor phase linearity. Furthermore, suchpass band resonator filters are (as noted above) too lossy in the passband and have insufficient amplitude flatness for a cellular radio basestation amplifier.

[0018] In another aspect, there is provided an LNA input stage includinga band reject filter comprising a shunt acoustic resonator and a seriesacoustic resonator, the shunt resonator being arranged to resonategenerally at the reject frequency band and the series resonator beingarranged to anti-resonant generally at the reject frequency band. Inthis configuration, forming part of an LNA input stage, the filter maybe used to reject power in the transmit band which is incident on theLNA.

[0019] In this way (as explained below), the transmit/receive isolationrequirements for the base station duplexer may be reduced, for example,from of the order of 90 dB to of the order of 45 dB. This allows asignificantly cheaper duplexer to be used for the base station.

[0020] Further, another aspect of the invention provides a duplexer fora mobile telephone handset including an RF band reject filter comprisinga shunt acoustic resonator and a series acoustic resonator, the shuntresonator being arranged to resonate generally at the reject frequencyband and the series resonator being arranged to anti-resonant generallyat the reject frequency band.

[0021] Such filters may also be used to reject aircraft bandinterference, TV or radio signals in a handset or base station. They mayalso be used to suppress an unwanted LO, sideband or image frequency ina TX or RX chain, where the distortion or loss introduced by a bandpassfilter would be unacceptable. Such a situation might arise in the caseof a transmitter which must carry a predistorted signal such as would bethe case in a system employing a baseband predistortion power amplifier.Since any filters would be within the correction loop, they must providevery little amplitude or phase distortion over the frequency ranges ofinterest.

[0022] Furthermore, since the equivalent circuit for a SAW resonator anda thin film bulk acoustic resonator (for example, the FBAR productavailable from Agilent) are identical, the invention may use a SAWresonator or a thin bulk acoustic resonator for the acoustic resonatorcomponent. One advantage of the thin film bulk acoustic resonatorimplementation is that the filter may be implemented completely insilicon which may, in some applications, provide packaging advantagesover a SAW resonator configuration.

[0023] Other aspects and features of the present invention will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic circuit diagram of a ladder configuration ofacoustic resonators in accordance with the invention;

[0025]FIG. 2 is a schematic representation of the circuit of FIG. 1 witha high-Q matching network;

[0026]FIG. 3 is a schematic block diagram of a cellular mobile basestation including filters in accordance with the invention;

[0027]FIG. 4 is a schematic diagram of a mobile handset including aduplexer in accordance with the invention;

[0028]FIG. 5 is a plot of simulated S21 parameters for a firstembodiment of the filter;

[0029]FIG. 6 is a plot of simulated S21 parameters for a secondembodiment of the filter; and

[0030]FIG. 7 is a plot of simulated S21 parameters for a thirdembodiment of the filter;

[0031] With reference to FIG. 1, two series resonators SR2 and SR4 areconnected in the series with an input port 6 and an output port 8. Twoparallel resonators PR10 and PR12 are connected in a ladderconfiguration between the series resonators SR4 and SR2 and a groundterminal 14.

[0032] The resonators may be SAW resonators or thin film bulk acousticresonators.

[0033] The series resonators SR4 and SR2 are designed to present a highimpedance at the reject band of interest. This is achieved by designingthe filters to be in anti-resonance at the reject band frequency. Shuntor parallel resonators PR10 and PR12 are designed to provide a lowimpedance at the reject band. This is achieved by designing thesefilters to be at or near resonance in the reject band.

[0034] Thus, the overall effect of the filter is to provide a highimpedance for signals presented between input terminals 6 and 14, at thereject frequency band.

[0035] In the applications which are explained in more detail below, oneof the desirable characteristics of the filter of FIG. 1 is the effectof the filter outside the reject band. In the pass band at frequenciesimmediately lower than the reject band, the series resonators SR4 andSR2 are at their resonant frequencies while the shunt resonators PR10and PR12 simply present a capacitive load to ground. In the pass bandimmediately above the reject band, the shunt resonators PR10 and PR12are at their anti-resonance frequency and hence present a high impedanceto ground while the series resonators SR2 and SR4 present a capacitativeseries impedance. At frequencies significantly above and below the passband, the series and shunt resonators simply behave as high Qcapacitors.

[0036] Thus, in the pass bands either side of the reject band, thefilter appears as a high Q series and shunt capacitor. The capacitor hasa very short delay in a very wide band characteristic with excellentphase linearity and excellent amplitude flatness. By incorporating highQ matching networks 16 and 17 as shown in FIG. 2, the insertion loss ofthe filter outside the reject band may be reduced to of the order of 0.2dB. The matching networks may be formed as serial and shunt passiveinductive and capacitive components as is know in the art.

[0037] optionally, an interstage shunt inductance 19 may be used to aidmatching. At frequencies of the order of 2 Ghz and above, matching maybe achieved using microstrip matching.

[0038] A symmetric response may be achieved using an optional additionalshunt resonator 21.

[0039] Although FIGS. 1 and 2 show a two stage ladder configuration, itwill be appreciated that a single stage (or more than two stages) may beused.

[0040]FIG. 3 shows an application of the type of filter shown in FIGS. 1and 2 in a base station for a cellular mobile radio network.

[0041] An antenna 20 is used to transmit and receive to and from mobilehand sets at different respective frequencies. A duplexer 22 is used tofeed signals in the transmit band, from a power amplifier 24 to theantenna 20 and to feed signals in the receive band from the antenna 20to a low noise amplifier 26.

[0042] A filter 28 used between amplifier stages 30 and 32 of the poweramplifier 24, is used to reject frequencies in the receive band. Thisreduces the energy present in the receive band at the input 34 to theduplexer.

[0043] Another acoustic resonator band reject filter 36 is tuned toreject the transmit band at the input stage to the low noise amplifier26.

[0044] By incorporating the filters 28 and 36 into the base stationdesign, the duplexer operating margins may be reduced since thesensitivity of the low noise amplifier to energy in the transmit band isreduced by the filter 36 and the likelihood of the power amplifier 24introducing energy in the receive band into the duplexer 22 is alsoreduced. Thus the cost of the duplexer 22 may be reduced which willresult in a significant cost saving in the overall cost of the basestation.

[0045] It will be appreciated that although FIG. 3 shows two amplifierstages 30 and 32 and a single interstage filter 28, the number ofamplifier stages and the number of interstage filters may vary from thatshown. Similarly, the representation of the band reject filter 36 andthe single LNA 26 is intended only to represent one possible example ofthe use of the filter of the present invention.

[0046]FIG. 4 shows a mobile telephone handset 40 which incorporates aduplexer which is similar in operation to the duplexer 22 of FIG. 3. Thepurpose of the duplexer is to allow simultaneous transmission andreception in different frequency bands using the same antenna whilstensuring that relatively high power transmit signals do not swamp thereceived signals which are to be amplified by the low noise amplifier.As noted above, SAW filters conventionally cannot be used for mobilehandset applications in the 1800 MHz or above bands because thecombination of high power and relatively high frequency rapidly destroysthe filter electrodes. Thus in practice, such duplexers are presentlybuilt from ceramic filters. However, since the resonant modes of thefilter of the present invention are in the receive band (when used inthe transmit circuit segment) the high powers do not damage the filter.Thus, both in the base station of FIG. 3 and the mobile handset of FIG.4, it is possible to use a SAW filter in the transmit circuit.

[0047] It will be noted, that the band reject filter may also be used toreject other sources of interference for example from the aircraft band.

[0048] The precise placement of the series resonator anti-resonancefrequency and shunt resonator resonance frequency can be adjusted toachieve a desired tradeoff between rejection band depth or rejectionband width. By placing the frequencies in close proximity to each otherthe rejection depth is maximized at the expense of the rejection bandwidth (FIG. 5) since the ratio of the series impedance to the shuntimpedance is maximized at that frequency. By offsetting the resonancefrequency of the shunt resonator slightly above (FIG. 6) or slightlybelow (FIG. 7) the anti-resonance frequency of the series resonator, therejection bandwidth can be increased at the expense of notch depth.

[0049] With reference to FIG. 8, the power handling capability of thisfilter may be increased using arrays of SAW filters 40 and 42. Eacharray consists of a plurality of series and parallel filters (in theexample shown a square array is used comprising nine filters arranged ina 3×3 grid). Assuming that parasitic capacitances are adequatelycontrolled, and assuming that each of the filters in each array isidentical, the frequency characteristic of the configuration of FIG. 8is identical to that of a single series and shunt resonator. However,since the voltages and currents across and through each resonator aredivided by virtue of the series and parallel configurationsrespectively, the overall power handling capability is greatlyincreased. Thus, for example, using a 3×3 array of resonators as shown,the power handling capability is increased by a factor of 9. This mayallow the band reject filter to be used at the power amplifier output ofa low power (e.g. 5 watt) micro base station, for example. Thisconfiguration may be replicated with more than two arrays.

[0050] For the avoidance of doubt, it will be noted that although theinvention is described above in connection with single port resonators,multiple port resonators (although believed not to be ideal) could alsobe used to implement the invention.

[0051] Thus, in summary, by building a band reject filter using acousticresonators, it is possible to use SAW resonator filters in applicationsfor which at least the insertion loss and power handling limitations ofconventional band pass resonator filters have proved unsuitable.

[0052] For the purposes of this specification, the term “RF” and “radiofrequency” are to be construed as limited to the range of frequencieswithin which SAW resonators, FBARs and any equivalent filters may bedesigned to operate. This is so because the operating frequency of theband reject filter of the present invention is limited only by thecapabilities of the resonators from which it is constructed.

[0053] It will be understood that the band reject filters describedabove can be designed on a large variety of substrates depending on theapplication and targeted specifications. It is will be appreciated bythose skilled in the art that the term SAW can be extended beyond theclassical Rayleigh wave and also encompasses Leaky SAWs, SurfaceSkimming Bulk Waves and Surface Transverse Waves.

[0054] The type of wave will depend on the substrate chosen. A fewexamples of substrates supporting the classical Rayleigh mode are STQuartz, YZ Lithium Niobate, 128 degree cut X-axis Lithium Niobate, X-cut112.2 Y-direction Lithium Tantalate to name a few. Examples ofsubstrates supporting leaky surface acoustic waves are LST Quartz, 64degree YX Lithium Niobate and 36 to 42 degree YX Lithium Tantalate.Examples of substrates supporting surface skimming bulk waves areRotated Y-cut ST-quartz, 35.5 degree (AT) rotated Y-cut quartz and 36degree rotated YX-Lithium Tantalate.

[0055] Furthermore, this device may be designed on multi-layersubstrates such as, but not limited to, Lithium Tantalate with an SiO2layer or a diamond substrate consisting of Si/Diamond/ZnO/Al/SiO2 suchas that presently being developed by Sumitomo.

What is claimed is:
 1. A radio frequency band reject filter comprising ashunt acoustic resonator and a series acoustic resonator, the shuntresonator being arranged to resonate generally at the reject frequencyband and the series resonator being arranged to anti-resonant generallyat the reject frequency band.
 2. A radio frequency filter according toclaim 1, comprising a plurality of shunt acoustic resonators eacharranged to resonate generally at the reject frequency band and aplurality of series acoustic resonators each arranged to anti-resonantgenerally at the reject frequency band, the shunt and series acousticresonators being arranged in a ladder configuration.
 3. A radiofrequency band reject filter according to claim 1, wherein the shunt andseries acoustic resonators are each formed as an array of a plurality ofserially and parallel connected resonators.
 4. A radio frequency filteraccording to claim 2, wherein the acoustic resonators are one portdevices.
 5. A radio frequency filter according to claim 2, wherein theacoustic resonators are surface acoustic wave resonators.
 6. A radiofrequency filter according to claim 2, wherein the acoustic resonatorsare thin film bulk acoustic resonators.
 7. A radio frequency filteraccording to claim 2, further including a high Q matching networkarranged to reduce the apparent capacitance of the filter outside thereject frequency band.
 8. A base station power amplifier for a cellularradio network, the power amplifier including at least one inter-stageband reject filter comprising a shunt acoustic resonator and a seriesacoustic resonator, the shunt resonator being arranged to resonategenerally at the reject frequency band and the series resonator beingarranged to anti-resonant generally at the reject frequency band.
 9. Apower amplifier filter according to claim 8, wherein the acousticresonators are one port devices.
 10. A power amplifier according toclaim 8, wherein the acoustic resonators are surface acoustic waveresonators.
 11. A power amplifier filter according to claim 8, whereinthe acoustic resonators are thin film bulk acoustic resonators.
 12. Aduplexer for a mobile telephone handset including an radio frequencyband reject filter comprising a shunt acoustic resonator and a seriesacoustic resonator, the shunt resonator being arranged to resonategenerally at the reject frequency band and the series resonator beingarranged to anti-resonant generally at the reject frequency band.
 13. Aduplexer according to claim 12, wherein the acoustic resonators are oneport devices.
 14. A duplexer according to claim 12, wherein the acousticresonators are surface acoustic wave resonators.
 15. A duplexeraccording to claim 12, wherein the acoustic resonators are thin filmbulk acoustic resonators
 16. A low noise amplifier input stage includinga band reject filter comprising a shunt acoustic resonator and a seriesacoustic resonator, the shunt resonator being arranged to resonategenerally at the reject frequency band and the series resonator beingarranged to anti-resonant generally at the reject frequency band.
 17. Aninput stage according to claim 16, wherein the acoustic resonators areone port devices.
 18. An input stage according to claim 16, wherein theacoustic resonators are surface acoustic wave resonators.
 19. An inputstage according to claim 16, wherein the acoustic resonators are thinfilm bulk acoustic resonators.